Many custom molders run high-mix, low-volume production schedules that require constant changeovers.
One consumer-products molder in the southeastern United States was performing more than 20 color changes per week while processing approximately four million pounds of polypropylene annually. Despite using a purge program, the company continued to struggle with lengthy hot runner color changes and excessive scrap generation.
Their challenge is not unique.
Color changes become increasingly difficult when processors run:
- Pearlescent colors
- Dark pigments
- Highly loaded color concentrates
- Appearance-critical applications
- Multi-cavity hot runner molds
In many cases, residual color remains trapped near gates and low-flow regions, making complete cleaning difficult.
The result is more startup scrap and longer periods before acceptable parts are produced.
Why Traditional Purging Approaches Often Fall Short
Many processors focus primarily on cleaning the screw and barrel during a changeover.
While this is important, it may not address contamination trapped inside the hot runner itself.
In some situations, molders continue cycling parts while waiting for old color to disappear. Others rely on extended flushing with production resin.
These approaches consume valuable machine time and often generate large amounts of scrap before stable production is achieved.
The challenge becomes even greater when processing teams need simple procedures that can be repeated consistently across multiple shifts and operators.
A Different Approach to Hot Runner Purging
Hot runner systems often require a purge strategy specifically designed for low-flow areas and difficult-to-clean manifolds.
Rather than treating hot runner cleaning as an extension of barrel cleaning, successful processors focus on cleaning the entire melt path.
This includes:
- Screw and barrel
- Nozzle
- Manifold
- Gates
- Hot runner drops
Because contamination frequently remains trapped near gates, selecting a purge compound designed for hot runner applications can significantly improve results. Some purge technologies are specifically engineered to perform in these difficult-to-clean areas while leaving minimal residue behind.
Real-World Results: Scrap Reduction of More Than 60%
The southeastern custom molder ultimately implemented a new hot runner purging strategy using a purging concentrate designed for challenging applications.
The results were dramatic.
Within two weeks, the company's hot runner scrap rate dropped from 5% to approximately 1.8%, representing a reduction of more than 62%. Color changes were completed in less than 15 minutes, allowing the team to spend more time producing good parts and less time generating waste.
Just as importantly, the process was simple enough to support a growing workforce without introducing additional complexity into production.
What Injection Molders Can Learn from This Case
The biggest lesson is that not all contamination originates in the barrel.
When color changeovers continue taking longer than expected, the hot runner system itself may be the source of the problem.
Processors experiencing recurring color contamination, excessive startup scrap, or long changeovers should evaluate:
- Whether contamination remains trapped inside the hot runner
- If the purge strategy is designed for low-flow areas
- Whether operators are cleaning the entire melt path
- If the purge compound is suitable for hot runner applications
Addressing these factors can often produce substantial improvements in both quality and productivity.
Conclusion
Hot runner systems are among the most difficult areas of an injection molding process to clean, which is why they frequently become a major source of scrap during color changes.
For processors running frequent changeovers, the costs can add up quickly through wasted resin, lost machine time, and extended startup periods.
By implementing a purge strategy specifically designed for hot runner systems, manufacturers can reduce contamination, shorten color changes, and significantly decrease scrap rates.
In many cases, improving hot runner cleaning is one of the fastest ways to improve overall molding efficiency.
A high-quality CPC should leave low residue to make for quick and efficient purging. Purging compound suppliers recommend different procedures for purging hot runners, and the method chosen depends largely on the resin(s) being processed, mold design and cleaning difficulty. If you have any questions, be sure to consult your supplier.
Learn how to reduce machine downtime with five quick purging compound tips for your injection molding application.
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